The two roller, tilting bridge type machine is suitable for the production of bodies of roatation, preferably tank bottoms fixed with radius, by metal spinning, without the need of costly metal spinning mushroom.
Earlier the tank bottoms were produced with hot pressing. In order to save the substantial technical expenditures necessary for the production of hot pressing equipment, the production of tank bottoms was started with new technology, i.e. metal spinning; however the large diameter required the use of large-size metal spinning mushroom.
In order to eliminate the use of the large-size metal spinning mushrooms, two processes were worked out.
According to one of the processes the large radius is formed on the pressing machine with a tool acting on a small surface in several steps, and the small radius is formed on another flanging press. The machines of the German firm Schleifenbaum and Steinmetz and of the Italian firm Boldrini are operating with such technology, where two machines and an additional costly manipulator for servicing the pressing machine are necessary due to the large dimensions and significant weight of the sheet metal discs.
According to another process the tank bottom is shaped between two rollers, and the shape of the inner roller corresponds to the flange of the tank bottom, i.e. to the small radius.
The machines according to Hungarian patent specification No. 153 215 and GFR patent specification Nos. DT 1 752 914 and DT 1 804 669 were designed for the latter process, where the inner shaped roller forms the large radius of the tank bottom moving together with the outer roller, then by the stepped movement of the inner roller, the outer roller shapes the flange of the tank bottom, i.e. the small radius by rolling the edge of the sheet metal onto the inner shaped roller. The radial centre is adjustable according to the large radius of the disc to be shaped, while the shape of the inner roller is selected according to the small radius.
Hungarian patent specification No. 175 686 is aimed at the elimination of the shortcomings of the machines described in the Hungarian patent specification No. 153 215 and GFR patent specification Nos. 1 752 914 and 1 804 669, according to which the inner and outer tilting bridges carrying the inner shaped and outer pressure roller, tilt around the centre of roation placed into the radial centre of the work piece and, the synchronous movement, to be disengaged upon flanging, is ensured with lock pins.
The machine produced according to Hungarian specification patent No. 175 686 proved the advantages, however since the bridges carrying the inner and outer rollers were shaped separately, the following two adverse phenomena came to light. One of them is that the compressive force arising from the rolling shaping becomes closed through the pin in the centre of rotation, consequently the outer and inner bridges, as well as the journal in the centre of rotation of the tilting, have to be dimensioned according to this force, which is 200-300 kN in case of an 18 mm steel plate. The other adverse phenomenon is that the synchronous movement of the inner and outer rollers has to be ensured with hydraulically actuated pins for forming the large radius, and these pins have to be disengaged, or the inner bridge is to be fixed prior to commencement of the flanging. Movement of the pins performing the fixing of the inner bridge and, thereafter, the disengagement of the pins ensuring the synchronous movement take time, during which the pressure roller, in lack of feed, presses the work piece along the penetration line of the large radius and small radius of the flanging, where it leaves a visible trace. A farther disadvantage of this solution resides in that the holes used for securing the inner bridge are fixed for each diameter, and the diameter of the work pieces can be corrected only by modification of the shaped inner roller.